Vehicles have been developed to automatically perform an idle-stop when idle-stop conditions are met and automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduction in exhaust emissions, reduction in noise, and the like.
Following an engine idle-stop, pressure may be maintained in the hydraulic line to enable transmission and driveline functionality and to reduce the time needed to restart the engine and engage the transmission clutches. Transmission hydraulic circuits may include an auxiliary transmission pump, such as an electrically-motored auxiliary pump, to pressurize fluid in the hydraulic circuit when the engine is shut-down and the mechanical transmission pump is not at full flow capacity. However, such auxiliary pumps may increase component costs while their operation may reduce fuel savings.
One example of maintaining hydraulic line pressure without using an auxiliary pump is illustrated by Tashiro et al. in JP8014076. Therein, when the engine is shut-down, the transmission is isolated and oil pressure in the transmission is maintained by a check valve and an accumulator. By maintaining hydraulic line pressure, transmission clutches may be engaged before the engine is restarted.
However, the inventors have recognized several potential issues with such an approach. As one example, the accumulator connected to the transmission clutches may reduce the hydraulic stiffness in the isolated transmission hydraulic circuit, for example, during adjustment of a transmission clutch during engine restarting. Consequently, during the clutch operation, the response time of clutch engagement may be substantially reduced. Additionally, the accumulator may cause pressure oscillations in the hydraulic circuit that may further delay clutch engagement at engine restart. As such, this may degrade the quality of an engine restart and vehicle launch operation.
Thus, in one example, some of the above issues may be addressed by a method of pressurizing a hydraulic circuit comprising a hydraulically actuated transmission component and an accumulator. In one embodiment, the method comprises, during an engine idle-stop, adjusting actuation of the hydraulically actuated transmission component over a duration, and during the duration, isolating the accumulator from the hydraulically actuated transmission component when a pressure in the hydraulic circuit is above a threshold, and coupling the accumulator into the hydraulic circuit when the pressure is below the threshold.
In one example, the actuation of a hydraulically actuated transmission component may be adjusted over at least a duration of an engine idle-stop operation. To provide the hydraulic line pressure needed to adjust the actuation of the transmission component during the engine idle-stop, a hydraulic circuit comprising the transmission component and an accumulator may be pressurized. In one example, the hydraulically actuated transmission component may be a transmission forward clutch. During an engine idle-stop, the forward clutch apply hydraulic circuit may be isolated from a transmission pump, for example, by closing a clutch isolation valve. As such, during the idle-stop, as the engine spins down and transmission pump flow capacity drops, hydraulic fluid may be exhausted (for example, to a sump) and hydraulic line pressure in the transmission may also drop. Herein, by isolating the clutch hydraulic circuit from the pump, hydraulic fluid and hydraulic line pressure may be held in the isolated clutch hydraulic circuit to facilitate clutch modulation.
During the engine idle-stop, while the forward clutch actuation is adjusted, an engine controller may be configured to isolate the accumulator from the transmission component when a pressure in the hydraulic circuit is above a threshold. In one example, the accumulator may be coupled via an accumulator isolation valve and isolating the accumulator may include closing the accumulator isolation valve. By isolating the accumulator, hydraulic stiffness may be maintained in the isolated hydraulic circuit. The engine controller may be further configured to compensate for pressure drops in the isolated clutch hydraulic circuit, for example due to leakages through clutch seals, circuit valves, and/or circuit solenoids, by selectively coupling the accumulator into the hydraulic circuit when the pressure drops below the threshold. The coupling may include, for example, opening the accumulator isolation valve. By coupling the accumulator, pressurized fluid may be delivered from the accumulator to the forward clutch to swiftly restore hydraulic pressure in the isolated circuit.
In one example, the forward clutch hydraulic circuit may remain isolated until a subsequent engine restart is completed and the transmission pump has resumed full flow capacity. Following engine restart, the forward clutch may be coupled to the transmission pump, for example by opening the clutch isolation valve, so that the pump may provide the forward clutch inlet pressure needed for clutch modulation. Additionally, the accumulator may be recharged, if needed, by opening the accumulator isolation valve and coupling the accumulator into the clutch hydraulic circuit.
In this way, hydraulic pressure may be maintained in a transmission clutch hydraulic circuit during an engine idle-stop, and/or until an engine restart is completed, without operating an auxiliary pump. Additionally, by alternately coupling and decoupling an accumulator to (and from) the transmission component, responsive to a pressure in the isolated clutch hydraulic circuit, hydraulic stiffness may be maintained in the circuit. By maintaining hydraulic line pressure in the isolated clutch hydraulic circuit, a transmission component inlet pressure, (e.g. forward clutch inlet pressure) may be modulated to enable clutch modulation over at least a duration of the engine idle-stop. For example, by adjusting the forward clutch inlet pressure, the forward clutch may be kept fully or partially engaged during the engine stop and/or until a subsequent engine restart is completed. Alternatively, the forward clutch may be kept fully disengaged with intermittent stroking. By maintaining the clutches in an engagement-ready state, and by enabling rapid response times, the transmission may be put in gear swiftly when an engine restart and/or vehicle re-launch is requested. Further, by maintaining hydraulic stiffness in the hydraulic circuit when the engine is shut-down, the response time for clutch engagement during the restart may be reduced and oscillations may be substantially averted. In this way, the quality of engine restarts may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.